Liquid crystal materials are of high commercial interest due to their beneficial use in electro-optical displays.
The electro-optical displays use electro-optical effects exploiting the electrical, respectively dielectrical, and the optical anisotropy properties of the liquid crystal materials. Thus, for application of liquid crystals to such effects, it is essential to assess besides the dielectrical properties of the liquid crystal materials, i.e. the dielectric permittivity parallel to the director (.di-elect cons..sub.ll), the dielectric permittivity perpendicular to the director (.di-elect cons..sub.l) and their difference, the dielectric anisotropy (.DELTA..di-elect cons.=.di-elect cons..sub.ll -.di-elect cons..sub.l), also their optical properties, i.e. the refractive index parallel to the director, called the extraordinary refractive index (n.sub.e), the refractive index perpendicular to the director, called the ordinary refractive index (n.sub.o) and their difference, the anisotropy of the refractive indices, called birefringence in this application (.DELTA.n=n.sub.e -n.sub.o).
Most widely used today are nematic liquid crystals consisting of calamitic, i.e., rod-like molecules which are characterized by an orientation of their long molecular axis which is on average parallet to a preferred direction. This preferred direction is generally referred to as the director.
Thus, in an oriented sample, nematic liquid crystals are characterized by anisotropic physical properties. i.e. their physical properties depend on the orientation relative to the director. Nematic liquid crystals generally are rotational symmetric to the axis of the director. This is called uniaxially anisotropic. Thus most of their physical properties, like e.g. refractive index, dielectric permittivity and magnetic susceptibility, adopt two different values when viewed either parallel or perpendicular to the director.
The expert has several methods available to determine the birefringence of liquid crystal media especially of nematic liquid crystal media.
Among the known methods are the use of the so called Leitz-Jelly refractometer and the use of refractometers exploiting the determination of the critical angle of total reflection, as for example the so called Abbe refractometer.
The refractometer according to Leitz-Jelly consists of a back plane comprising a slit and a measuring scale and of a sample holder. The sample holder features a wedge-shaped space between a flat transparent body and a prism. This prism is located on the opposite side of the back plane. On the side of the flat transparent body of the sample holder which faces the back plane a nontransparent plate with a narrow hole is affixed.
The liquid crystal is oriented between the flat transparent body and the prism. When it is now illuminated from the back, the light passing through the slit in the back plane and through the small hole in the nontransparent body behind the sample holder passes the sample being split into an ordinary and an extraordinary beam.
The observer, located at the prism side of the sample holder can then determine the imaginary origins of both beams on the graded scale of the back plane.
The use of the Leitz-Jelly refractometer is briefly explained e.g. in "Ekisho Kisohen" by K. Okano and S. Kobayashi. Kabushiki Kaisha Baifukan (Jul. 15, 1985 and Jun. 15, 1989). ISBN-563-03414-2, chapter 10.3.1, pp. 212-213.
The use of the Abbe refractometer is e.g. described in "Optical anisotropy" by U. Finkenzeller and R. E. Jubb, part IV of "Physical properties of liquid crystals", status November 1997, Ed. W. Becker, Merck KGaA, Germany.
These methods, especially the latter one, have the advantage that both the ordinary and the extraordinary refractive index and thus also their difference, the birefringence, can be determined almost at the same time for one and the same sample. Thus there is no uncertainty about any change in conditions which would influence the refractive indices, like the orientation of the sample and the temperature of the sample. In fact, by the method using the Abbe refractometer the ordinary and the extraordinary refractive index of the material are determined not simultaneously but subsequently only, but a change in the boundary conditions and in the surroundings can be practically excluded by repetition of the previous readings. Thus this method is a "quasi-simultaneous" measurement method.
The method using a Leitz-Jelly refractometer allows the determination of a wide range of both n.sub.e and n.sub.o values and thus also of .DELTA.n values, however the accuracy of this method is only about .+-.0.001 for both n.sub.e and n.sub.o and thus only .+-.0.002 for .DELTA.n.
Consequently for most practical applications the method using the Abbe refractometer, having, at least ideally, an accuracy of .+-.0.0002 for the refractive indices n.sub.e and n.sub.o and of .+-.0.0004 for the birefringence .DELTA.n is preferred.
Problem to be Solved by the Invention
The Abbe refractometer is one example of refractometers employing the principle of the critical angle of total reflection between two optically transparent media with different refractive indices. Thus, naturally but unfortunately, the range of refractive indices accessible using this refractometer, like others based on the same measuring principle, depends on the refractive index of the measuring body used, which typically is a measuring prism.
The total range of refractive indices accessible by commercially available Abbe refractometers (e.g. by Zeiss, Germany) ranging from 1.17 to 1.85 is already rather wide, but it is only accessible with three different measuring prisms. One prism allowing measurements in the range of 1.17 to 1.56, the next one from 1.30 to 1.71 and the last one from 1.45 to 1.85 (Abbe refractometer B, manual of operation, Zeiss, Germany).
Thus refractive indices of compounds varying over a wide range of values can only be determined using different measuring prisms. On the one hand, exchanging the prisms of a refractometer is rather economical, however it takes an appreciable amount of time, especially considering the time required for thermostating the different prisms. It also increases the risk of damaging the prisms due to more frequent handling and exposure. On the other hand, the parallel use of different refractometers is more time efficient, but less economic.
Further, recently there is an increasing demand for liquid crystal materials with large birefringence values even with .DELTA.n&gt;0.4. In order to determine the birefringence of such materials, as well as e.g. the birefringence of materials with n.sub.o in the range from 1.3 to 1.45 and .DELTA.n in the range from 0.25 to 0.40, it is necessary to conduct two separate measurements of n.sub.e and n.sub.o either changing the measuring prisms of the refractometer between measurements or using two refractometers with different prisms. Both these methods, however, lead to the loss of the benefits of the simultaneous or almost simultaneous measurement.
Thus, uncertainties due to possible differences in measurement conditions, especially such as orientation of the sample and measurement temperature, lead to a smaller accuracy and even to systematic errors.
Thus it is highly desirable to maintain the benefits of the measuring method using e.g. the Abbe refractometer, i.e., the good measurement accuracy and the at least quasi simultaneous measurement and at the same time to significantly enlarge the accessible range of refractive indices.